1. Field of the Invention
The present invention relates generally to pumps and more particularly, to a pressure relief arrangement for pumps.
2. Description of Related Art
Normal water pumps do not handle solids but it has been noted that when the flowrate is say, equal to or less than 10% than that of the maximum flowrate at any particular pump speed, the temperature of the liquid recirculating inside the pump will increase with time. The heat generated causes the pump casing and components to also increase in temperature. It is therefore quite common for manufacturers to recommend a minimum flowrate for a pump to avoid this problem area. Measurement and control of flowrate and therefore temperature for water pumps are relatively easy and there is a multitude of suitable equipment available. Some schemes involve a separate bypass to maintain flow through the pump.
Centrifugal Slurry Pumps are typically applied in a very wide range of industries and applications worldwide and most commonly in mining plants. The mixture of liquids (commonly water) and solids that make up the slurry that these slurry pumps handle are also very wide ranging. Similar to water pumps, slurry pumps will heat up if operated at low flowrates for any significant time. Low flow rates can be caused inadvertently by blockages occurring in the pump due to the slurry being pumped. The heat generated can also be detrimental to the wear resistant hard metal or natural rubber liners commonly used in slurry pumps. In a worst case scenario, it is possible that the steam generated from such overheating due to pump blockage conditions may cause the pump to explode.
Slurry pumps are normally installed in quite similar types of arrangements, with a hopper to gravity-feed the slurry into the pump, followed by different length pipelines, generally with bends, sloping or horizontal sections of pipework. In some cases valves or tanks are located along the pipeline to the final discharge point.
For measuring slurry flowrate or slurry fluid temperature, there are relatively few options available since slurry can easily clog or jam instruments and/or cause wear. Consequently, it is common practice to utilize very few instruments in the pumping of slurry and to rely on the continuous flow of slurry from one process to another. Slurry pump manufacturers and suppliers can provide a minimum flowrate for a slurry pump, but with the wide range of possible duties, change in slurry properties and the possibility of solids settling in the pipeline or pump, such minimum flowrate recommendations will not, by themselves, guarantee that the flowrate will not change or drop in service to critically low levels.
Transport of the slurry particles relies on maintaining a certain velocity in the pipeline; otherwise particles tend to settle out on the bottom of the pipe. As the velocity drops further, the solids will build-up in the pipeline and eventually may cause a blockage. A similar scenario can occur in a slurry pump operating at very low or zero flowrate. The solids start to settle out in the pump and can cause a blockage. Even if the pump is running, the pump can eventually become completely choked with solids.
All horizontal slurry pumps have a pump casing with an impeller rotating inside the casing. The impeller is attached to one end of a cantilevered shaft. The shaft rotates in bearings and enters the drive side of the pump casing through a seal chamber that houses a seal device of some form. The seal chamber is normally a separate component that is positioned at the back of the pump casing and takes a number of forms. One form is a stuffing box, which contains packing rings that provide a seal device for sealing the shaft as it passes through the seal chamber/pump casing wall. Another form is an expelling chamber. One or both of these two forms can be utilized regardless of the pump duty, liner material or application. Another type of seal device is a mechanical seal. In all cases, the seal device is contained in the seal chamber, which is supported by the pump casing.
The seal chamber at the drive side of the pump is supported by the pump casing and is generally sealed at its periphery against the internal pump liner, which could be metal or elastomer material. The internal pressure inside the pump casing acts on the inside surface of the seal chamber. The seal chamber is sealed against the main pump liner with a seal such as an O-ring seal or other type of elastomer seal.